1. Field of the Invention
The present invention relates to a navigation device and a navigation method capable of route search and route guide for a vehicle in a fuel-efficient manner.
2. Related Art
A navigation system searches a route from a departure point to a destination and proposes the searched route to a driver. Usually, in the route search of this kind, road links, each having a some kind of predetermined cost (hereinafter called a link cost) and forming a probable route between the departure point and the destination, undergoes calculation using a cost function based on its link cost of each road link so that the result in the calculation of the cost function should be minimized.
Usually, the cost function used in a navigation system of this kind is a time necessary to travel a route between the departure point and the destination; or the distance (length) of the route. That is, the link cost set for each road link in this case is a time necessary to travel the route or the length of each link. The navigation system uses a predetermined algorithm and searches a time-saving route or a distance-saving route, which has the result calculated and minimized by using the cost function.
A cost function for route search may be amount of fuel to be consumed (hereinafter called fuel consumption). The link cost of each road link in this case must have a predetermined fuel consumption of a vehicle which will travel the road link. The navigation device can search the fuel-efficient route, which can minimize fuel consumption, by using the link costs each having a predetermined fuel consumption.
The fuel consumption of the vehicle which will travel each road link must be measured or estimated prior to setting the fuel consumption as a link cost of each road link. The following patent documents disclose examples of navigation devices which search the fuel-efficient route by measuring or estimating fuel consumption of each road link.
For example, Japanese Patent Laid-open Publication No. 2006-098174 (hereinafter called Patent Document 1) discloses a technique of preparing correlation among geometry, congestion condition, and fuel efficiency in advance; and estimating fuel consumption of each road link based on a three-dimensional road map and traffic condition of each road link. Japanese Patent Laid-open Publication No. H02-278116 (hereinafter called Patent Document 2) discloses a method of estimating fuel consumption of each road link based on a record of fuel consumption obtained in the past. Japanese Patent Laid-open Publication No. H10-122883 (hereinafter called Patent Document 3) discloses a method of estimating fuel consumption of each road link by multiple regression analysis using a record of fuel consumption obtained in the past and various parameters, indicative of road conditions, such as the length of each link, the number of traffic signals, the number of traffic lanes, and altitude above sea level etc.
In the invention disclosed in Patent Document 1, a three-dimensional road map is indispensable for estimating the fuel consumption of each road link accurately. Preferably, such a three-dimensional road map should include information indicative of ups and downs in each road link in addition to the difference of altitude from sea level between the beginning point and the ending point of each road link. However, a large-capacity storage device is necessary for storing such a three-dimensional road map which usually has a considerable data size. A large-capacity storage device is an unbearable burden for an on-board navigation device.
Patent Document 2 discloses a method not requiring a three-dimensional road map, but a link cost indicative of fuel consumption of a road link can be obtained unless the vehicle has record of travel. In contrast, the invention disclosed in Patent Document 3 can estimate fuel consumption of a road link for which the vehicle has a record of travel. However, the information indicative of ups and downs in each road link or congestion information can hardly be reflected on data of the fuel consumption in this case.
In addition, no prior art considers a capability of navigation system for obtaining various fuel consumption which varies in accordance with various type of vehicles each having different vehicle weight and fuel efficiency. For example, Patent Document 1 does not disclose a method of estimating a link cost indicative of fuel consumption for each type of vehicle.
In the invention disclosed in Patent Document 2 or 3, a set of data indicative of fuel consumption of various type of vehicles travelling various type of road links must be obtained prior to calculating a link cost indicative of fuel consumption or obtaining parameters used in multiple regression analysis. However, a solution for that purpose will be very costly and time-consuming.